Double receptacle filtration system

ABSTRACT

A faecal filter assembly comprises two open-mouthed receptacles adapted to be joined by a hollow stopper. A tubular filter extends centrally from the stopper so that the latter provides a shoulder at the open end of the receptacle where debris and the like will collect. The arrangement is such that when the assembly is placed in a centrifuge, centrifugal forces will not force debris through the filter but instead will collect it on the shoulder. Preferably the filter is a lattice of oppositely-extending ribs defining between them pores.

FIELD OF THE INVENTION

This invention relates to improvements in filters.

BACKGROUND OF THE INVENTION

It is conventional to examine a biological sample, for example a faecalsample, by placing it in a first tubular receptacle which is then closedby a filter medium. The first tubular receptacle may be attached to asecond one in axial alignment such that the sample will pass into thesecond receptacle from the first through the filter. Filtration can beachieved simply by standing the assembly with the first receptacleuppermost or it may be encouraged by shaking the joined receptaclesand/or placing them in a centrifuge. Following the completion offiltration, particles trapped by the filter and/or precipitated to theclosed end of the second receptacle can be removed for analysis.

A double-receptacle filtration system of this kind is particularly,although not exclusively, suitable for the treatment of faecal sampleswhere it is desired to isolate and remove for analysis of parasites,their eggs and larvae which may be present in the sample. The handlingof such samples is unpleasant and presents biological hazards so thatthe “closed” double-receptacle filtration system is particularlyattractive. Hitherto, however, the filter medium used has most commonlybeen a disc of woven material or a molded lattice which can be fittedacross the open mouth of the first receptacle. This is liable to becomeclogged and occluded by larger particles suspended in the sample, suchas pieces of undigested food Moreover if the double-receptacle is placedin a centrifuge this will tend to drive particles through the filterinto the second receptacle. For this reason filtering and centrifugingare normally done separately and this two-stage operation is timeconsuming and labor intensive, as well as representing a hazzard to theoperator when the two receptacles are disconnected.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a filtrationassembly comprising a sample mixing receptacle closed at one end andopen at its other end, a second receptacle having a closed end and anopen end and adapted to be fixed in coaxial alignment with the mixingreceptacle to receive filtrate from the latter when the two, joinedreceptacles are placed in a centrifuge and a stopper for the open end ofthe mixing receptacle, the whole assembly being adapted to be placed ina centrifuge, wherein a tubular filter extends from the center of thestopper such that when the mixing receptacle is stoppered a closed endof the filter confronts the closed end of the receptacle and an open endof the filter opens through the stopper, the stopper providing ashoulder around the filter at the open end of the mixing receptacle.

The tubular filter provides an enlarged filter area by comparison with adisc filter, but a more important advantage is that centrifugual forces,instead of driving particles through the filter will instead drive themoff the filter and toward the shoulder, where they will be retained whenthe two receptacles are subsequently disconnected. This means thatfiltration and centrifugeing can be carried out as a single operation.There is less danger of contamination of the sample in the secondreceptacle and the two receptacles can be disconnected without exposingthe operator to the residue in the first receptacle.

Preferably the stopper is adapted to make screw-threaded connections toboth of the receptacles and means is provided whereby the secondreceptacle is preferentially unscrewed from the stopper when the tworeceptacles are oppositely twisted.

The filter preferably comprises a lattice structure of crossing,elongated elements of which at least those presented outwardly of thefilter tube extend outwardly of the filter with respect to the plane ofthe pores formed by the crossing elements, thereby to provide ribs onthe exterior of the lattice which will act as a pre-filter for largerparticles in advance of said pores. The interior of the filter tube ispreferably additionally ribbed, the internal ribs extendinglongitudinally of the filter tube and the external ribs extendingtransversely thereof.

It is known that tubular filters can be made by molding a plasticsmaterial into a lattice formation. However the porosity of such a“lattice” is considerably inferior to that of a woven filter, i.e. thepores of a molded filter are relatively few in relation to the totalfiltration area, so that it is not apparent that this alternative wouldbe an improvement.

In accordance with a preferred embodiment of the present invention thefilter comprises a lattice structure of crossing, elongated elements ofwhich at least those presented outwardly of the filter tube extendoutwardly of the filter with respect to the plane of the pores formed bythe crossing elements, thereby to provide ribs on the exterior of thelattice which will act as a pre-filter for larger particles in advanceof said pores.

Preferably the interior of the filter tube is additionally ribbed, theinternal ribs extending longitudinally of the filter tube and theexternal ribs extending transversely thereof.

An advantage of this arrangment is that occlusion of part of the lengthof a channel defined by two adjacent ribs by a large particle will notocclude the pore or pores confronted by the trapped particle, becauseliquid will still reach said pore or pores along the channel beneath theparticle. It has been calculated that particles just large enough to betrapped by the ribs can confront each pore within the lattice withoutany observable restriction of the flow of liquid through the filter. Ifthe depth of the ribs is increased then larger particles can confrontseveral pores within the lattice without any observable restriction inthe flow through the filter.

In a molded, tubular construction any internal ribs must be longitudinalas otherwise it would be impossible to remove the mould core.Effectively this means that any external ribs must be transverse (theouter mould part being separated into two halves to remove it from themolded filter). Transverse external ribs on a tubular filter formationhave a dual advantage. During manufacture and while the filter is stillcontained in the external mould part they serve to anchor it while themould core is withdrawn from the interior of the filter. When the filteris in use the fact that the external ribs are transverse means that thechannels formed between them are at right angles to the “natural flow”of the liquid sample out of the first receptacle. Causing the liquid tochange direction to enter the channels enhances filtration by creating agreater tendency for larger particles to be trapped by the “pre-filter”.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a sectional elevation of a faecal filter centrifuge tubeassembly in accordance with the invention, and

FIG. 2 is an enlargement of the area of the filter identified at “A” inFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The faecal filter illustrated comprises two similar, open-mouthedreceptacles 10 and 11 adapted to be joined together mouth-to-mouth inaxial alignment as illustrated. This assembly of the two receptacles 10and 11 is adapted for placing in a centrifuge.

Prior to joining the two receptacles together a faecal sample is placedin the first receptacle 10, which is then stoppered by screwing into itsmouth a hollow stopper assembly 12 from the center of which there thenextends longitudinally within the receptacle 10 a tubular filter 13.This has a closed end 14 confronting the closed end 15 of receptacle 10.Its other end opens through stopper 12, which extends as an annularshoulder or flange 20 radially from the mouth of filter 13.

Tubular filter 13 is manufactured as a lattice of crossing, elongatemembers 16 and 17. The members 16, which are internal and extendlongitudinally of the tubular filter 13, are offset from the members 17,which are external and extend transversely of the tubular filter 13, onopposite sides of the plane of the pores 18 which are bounded by themembers. Thus the members 16 form longitudinal internal ribs of thefilter while members 17 form transverse external ribs. Thus on bothsides of the filter there are channels between parallel ribs 16 or 17through which liquid will pass before passing through the pores 18.

The internal ribs 16 must be generally longitudinal of the filter 13 toenable it to be made in a moulding process from a plastics material, asotherwise the internal mould core (not shown) could not be withdrawn.The transverse arrangement of the external ribs 17, however, presents nosuch problem as two mould halves (not shown) of the external part of themould can be separated to allow removal of the molded filter. Prior tosuch removal, however, the transverse ribs 17 allow the external mouldpart to grip the tube while the mould core is removed.

It will be seen that each rib 16 or 17 is of trapezium cross section andthat its depth is greater than the width of each pore 18. The greaterthe depths of the channels, formed between parallel ribs 16 or 17, theless will be the tendency of any particle occluding a channel to occludethe pores in the bottom of the channel.

In use of the apparatus illustrated a faecal sample is placed in thereceptacle 10, which is then stoppered with the filter assembly 12 and13. When filtration is to be carried out the two receptacles 10 and 11are joined mouth-to-mouth as shown and liquid is allowed to pass fromreceptacle 10 through the filter 13 into receptacle 11.

Particles too large to pass through the pores 18, for example pieces ofundigested food, will tend to lodge across the ribs 17, therebyoccluding part of the length of a channel between two adjacent ribs 17but not occluding the pore or pores 18 immediately below the particle,because liquid can still reach such pore(s) along the length of thechannel.

Filtration may be assisted by shaking the assembly of receptacles 10 and11 and/or placing it in a centrifuge. Thereafter substantially all ofthe liquid formerly in receptacle 10 will have passed to receptacle 11and any parasites, their eggs and/or larvae will collect at the closedend 19 of receptacle 11. As is known per se, after separating the tworeceptacles 10 and 11 the parasites may be removed from receptacle 11 bypipette for analysis. In this process the receptacle 10 is preventedfrom unscrewing from the hollow stopper 12 by a ratchet/frictionmechanism such that the two parts are removed together. Meanwhile assoon as flow through the filter 13 has ceased particles trapped by theribs 17 will fall to the shoulder 20 provided by the stopper element 12,or may be assisted to do so by tapping the receptacle 10.

What is claimed is:
 1. A filtration assembly comprising: a sample mixingreceptacle closed at one end and open at its other end, the samplemixing receptacle provided with a stopper for the open end; and a secondreceptacle having a closed end and an open end, the second receptacleadapted to be fixed in coaxial alignment with the mixing receptacle toreceive filtrate from the sample mixing receptacle, wherein a tubularfilter extends centrally from the stopper such that when the samplemixing receptacle is stoppered a closed end of the tubular filterconfronts the closed end of the sample mixing receptacle and an open endof the tubular filter opens through the stopper, the stopper providing ashoulder around the tubular filter at the open end of the sample mixingreceptacle; wherein the closed end of the mixing receptacle and theclosed end of the second receptacle exclude any flow passages.
 2. Theassembly as claimed in claim 1, wherein the stopper is adapted to makescrew-threaded connections to both of the receptacles.
 3. The assemblyas claimed in claim 1, wherein the tubular filter comprises a latticestructure of crossing, elongated elements of which at least thosepresented outwardly of the tubular filter extend outwardly of the filterwith respect to a plane of pores formed by the crossing elongatedelements, thereby providing ribs on the exterior of the lattice whichwill act as a pre-filter for larger particles in advance of the pores.4. The assembly as claimed in claim 3, wherein the lattice structure ofcrossing, elongated elements comprises internal ribs extendinglongitudinally in a direction from the closed end of the tubular filterto the open end of the tubular filter and external ribs extendingtransversely to the internal ribs.
 5. The filtration assembly accordingto claim 1, wherein the tubular filter comprises a lattice structure ofcrossing elongated elements, including (i) internal ribs extendinglongitudinally in a direction from the closed end of the tubular filterto the open end of the tubular filter and (ii) external ribs externalribs extending transversely to the internal ribs.
 6. The filtrationassembly according to claim 5, wherein the internal ribs and externalribs define a plurality of pores, whereby particles trapped by theexternal ribs can confront one or more pores without any observablerestriction in the flow of liquid through the tubular filter.
 7. Afiltration assembly adapted for placing in a centrifuge, the assemblycomprising: a sample mixing receptacle closed at one end and open at itsother end; a stopper for the open end of the sample mixing receptacle;and a second receptacle having a closed end and an open end and adaptedto be fixed in coaxial alignment with the mixing receptacle to receivefiltrate from the latter, wherein a tubular filter extends centrallyfrom the stopper such that when the sample mixing receptacle isstoppered a closed end of the tubular filter confronts the closed end ofthe sample mixing receptacle and an open end of the tubular filter opensthrough the stopper, the stopper providing a shoulder around the tubularfilter at the open end of the sample mixing receptacle, and wherein thetubular filter comprises a lattice structure of crossing, elongatedelements of which at least those presented outwardly of the tubularfilter extend outwardly of the tubular filter with respect to poresformed by the crossing, elongated elements, thereby to provide externalribs on the exterior of the lattice structure which will act as apre-filter for larger particles in advance of the pores.
 8. The assemblyas claimed in claim 7, wherein the lattice structure comprises internalribs extending longitudinally of the tubular filter and the externalribs extend transversely to the internal ribs.